The goal of manifesting is to change the nature of your reality to match the desire being created. Thus, the primary goal wished to be achieved in manifesting is the final result of change. Since the nature of your reality is that of a vibrational reality, the primary goal in manifesting thus becomes that of changing the vibration of your reality.
The frequency of the vibration of your reality is made apparent through your thoughts and feelings. The more desirable are your thoughts and feelings on average, the more desirable is the vibration of the reality which is being experienced. Therefore, as the nature of your thoughts and feelings change, so changes the nature of the reality being experienced.
Emotions thus become the very perception of a change in the vibration of your reality since stronger emotions indicate your exposure to a vibration that differs greatly from the previously experienced vibration that was encountered in the moment prior to its occurrence. Since the degree of emotion being experienced indicates the degree to which your reality is changing, it may thus be used to gauge the degree to which your reality is changing in any given moment.
By thinking thoughts that produce particularly strong positive emotions, you can generate a considerable amount of change within your reality. Furthermore, the nature of this change is also indicated by the nature of the emotions being experienced since your emotions are the vibrations that are averaging with the vibration of your reality. Your reality is thus changing to better match the vibrations that are experienced as emotions.
As your reality becomes more alike in nature to these vibrations, you will then experience those particular vibrations more extensively, as is perceived in the repeating of the same emotion that created that particular state of reality. Ergo, as you choose to think thoughts that produce a certain emotion within you, that emotion will continue to be experienced going into the future, and it will actually become amplified with time as the vibration of your reality becomes increasingly harmonized with its frequency.